Determining the position of a cylinder piston as exactly as possible is a prerequisite for a number of technical applications, such as for example the automated actuation of a piston-cylinder unit of a construction machine or a hoisting implement by using the exact piston position as control variable. In addition, determining the position can contribute to an increase of the operational safety, as the inlet of the hydraulic medium, in particular the hydraulic fluid, in extreme positions of the cylinder piston must be controlled exactly and hence be stopped in time.
Beside the conventional measurement methods, which determine the current piston position by arranged cable-pull potentiometers or on the basis of a magnetostrictive principle, further measurement methods have emerged, which provide for determining the piston position on the basis of the electrical properties of the piston-cylinder unit. When the current piston position changes, this will influence the intrinsic electrical properties of the cylinder. The evaluation of the electrical properties is effected by an evaluation unit which is connected with the piston rod and the cylinder jacket and possibly operates with high-frequency signals.
At present, grounding of the piston rod is practiced at its extended ends, i.e. in the region of the piston rod eye at which the introduction of force into the mechanism is started. Via its length, however, the piston rod portion extended from the piston acts as antenna structure, whereby disturbing influences such as for example EMC, EMI from the environment act on the position determining system. Especially when using a measurement method on the basis of high-frequency evaluation signals, disturbing EMC, EMI effects can influence the accuracy of the position determining system. The mode of operation of surrounding electric and electronic components likewise can be influenced by a high-frequency radiation.
Therefore, it is the objective of the present disclosure to develop a piston-cylinder unit with such position measuring device, which reduces the influence of problematic signal couplings into and out of the piston rod.
This object is solved by a piston-cylinder unit with a cylinder jacket and a piston rod longitudinally shiftably guided in the cylinder jacket. The operating principle of the piston-cylinder unit can be pneumatic or hydraulic. The piston-cylinder unit comprises an evaluation unit which determines the position of the piston rod with respect to the cylinder jacket on the basis of the intrinsic electrical properties of the piston-cylinder unit. For this purpose, the evaluation unit on the one hand is connected with the piston rod and on the other hand with the cylinder jacket and can determine the intrinsic electrical properties of the piston-cylinder unit in dependence on the piston position via a corresponding evaluation electronic.
According to the present disclosure, the piston rod is short-circuited against the cylinder or the cylinder head in a manner suitable for high frequencies via one or more contact elements in the region of the exit opening of the piston rod out of the cylinder head. By early grounding of the piston rod, i.e. in the region of the exit opening, the influence of the extended piston rod acting as antenna structure is suppressed. Furthermore, the influence of the emission of the produced measurement signals of the evaluation electronic on the surrounding electric and/or electronic components can be reduced.
Grounding can be effected via a contact of the cylinder head with the cylinder, when the cylinder is connected with the machine chassis due to its mounting position. In this way, a contact of the piston rod to the cylinder potential very effective in terms of high frequency is achieved, whereby improvements in the suppression of emitted and irradiated electromagnetic signals are obtained. This increases the insensitivity to EMI and reduces the electromagnetic radiation of the active evaluation electronic.
Grounding can be effected via a conductive connection between piston rod and cylinder head, for example by one or more sliding contacts etc. In this case, it is conceivable that the evaluation electronic is at least partly arranged within the cylinder head and is connected with the piston rod and the cylinder jacket. The connection/contacting of the evaluation electronic with the piston rod at least should be effected within the cylinder head, i.e. between short-circuit contact and piston.
The short circuit caused by the grounding according to the present disclosure possibly can be prevented by the intrinsic evaluation of the piston-cylinder unit. This short circuit may be resolved by including at least one shielding impedance, which is connected in parallel with the intrinsically electrical properties of the piston-cylinder unit. In other words, the at least one shielding impedance is connected in series with the short-circuit path, i.e. in series with the piston rod and the short-circuit contact. By introducing the series impedance, the short-circuit branch formed by grounding the piston rod can again be resolved in terms of high frequency.
The at least one shielding impedance may include a frequency-dependent impedance. The shielding impedance can comprise a shielding ring of a permeable material, which is coaxially seated on the piston rod. In an example aspect of the present disclosure, the shielding impedance is arranged between the connection of the evaluation unit to the piston rod and the short-circuit contact, whereby a series connection of evaluation unit and shielding impedance to the grounding or cylinder potential is obtained.
It is possible to galvanically connect the evaluation unit with the piston rod. Alternatively, a capacitive connection of the evaluation unit is possible. In this case, the connection of the evaluation unit can be effected by a dielectrically insulated counterelectrode, which is guided concentrically or partly concentrically around the piston rod. The counterelectrode may be galvanically connected with the evaluation unit.
When the shielding impedance is connected in series with the evaluation unit, small changes in resistance have less influence on the measurement. The system thereby is shielded such that a higher impedance during the connection, in particular during a capacitive coupling with the piston rod, is admissible. In the case of the capacitive coupling, a tolerable higher impedance would allow a reduction in surface area or volume of the connection, which involves certain advantages with regard to the narrow space conditions within the cylinder head.
In an example aspect of the present disclosure, the evaluation unit of the piston-cylinder unit determines the current piston position on the basis of the inductive and/or ohmic properties of the conduction path formed between cylinder jacket, piston and piston rod. It therefore is expedient when the piston and the cylinder jacket are conductively connected with each other by an electrically conductive piston ring or sliding contact. The piston ring or sliding contact for this purpose is arranged between piston and cylinder jacket.
The evaluation of these inductive and ohmic variables can be effected by evaluation methods based both on alternating voltage and on direct voltage. For this purpose, the evaluation unit is provided with a corresponding evaluation electronic.
Alternatively, it is possible that the evaluation unit determines the piston position on the basis of the intrinsic capacitance of the capacitor formed by the cylinder jacket, the piston rod and a medium acting as dielectric within the cylinder jacket. The evaluation unit uses an oscillator circuit for the capacitance determination of the capacitance formed of piston and cylinder jacket.
Furthermore, it is possible that the evaluation unit determines the resonance behavior of the resonant circuit formed of cylinder jacket, piston rod and piston. The necessary capacitance of the resonant circuit is formed by the cylinder jacket and the piston rod, wherein a medium contained in the cylinder jacket, for example hydraulic oil, acts as dielectric. The conduction paths or the evaluation electronic act as inductance of the resonant circuit. For example, by excitation by an oscillator a vibration is produced within the resonant circuit formed, which is evaluated correspondingly via the evaluation unit.
Beside the piston-cylinder unit according to the present disclosure, the present disclosure relates to machine or working machine, in particular construction machine, hoisting implement, industrial machine, machine tool or aircraft with a corresponding piston-cylinder unit according to the present disclosure. The machine obviously has the same advantages and properties as the piston-cylinder unit according to the present disclosure, which is why a repetitive description will be omitted at this point. Inside an aircraft, the piston-cylinder unit according to the present disclosure may be used for the landing gear system or flight control actuators.
Further advantages and properties of the present disclosure will be explained in detail below with reference to the exemplary embodiment illustrated in the Figures. Note also that the inventors herein have recognized the above issues, which are not admitted to be well known.